The embodiments of the present invention relate generally to the detection, use and/or mitigation of multipath signals for communication and especially navigation systems. Certain embodiments further relate to the detection, use and/or mitigation of multipath signals for GNSS systems.
Wireless communications systems generally experience an effect known as “multipath”. “Multipath” refers to the reception of a non-line of sight signal from a signal source. Multipath signals may result, for example, from the reflection of a signal from a nearby reflector, such as the ground, a building face or the surface of a body of water. Multipath signals may also result when signals are significantly refracted. In general a non-reflected or refracted (straight line) signal is referred to simply as the “signal”, “true signal” or “line of sight signal”, whereas a reflected or refracted signal is referred to as a multipath signal. Since the multipath signal does not travel along the line of sight, it always arrives at the receiver later than the true signal.
Multipath signals, although sometimes useful, are often detrimental to signal quality. As an example, if the chipping frequency of a ranging signal is approximately 1 MHz, a multipath signal that travels along a reflected path with an additional 300 m length will arrive one chip later. When superimposed on the true signal, it distorts the received signal quality and can in some cases result in a complete loss of signal.
Multipath problems are also detrimental to navigation systems, especially those that rely on the time of reception of received signals, such as used in Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS) or Galileo. FIG. 1, although an illustration of embodiments of the invention, and not of prior art, can help illustrate the detrimental effect of multipath signals in navigation schemes that rely on the time of reception of a signal.
FIG. 1 illustrates a two-dimensional satellite navigation case. A receiver 100 is located at a point 100 on the surface 110 of the Earth, which is marked in this case with the arrow indicating the “true position” of the receiver 100. The receiver 100 receives signals from satellites 102 and 104. In the two dimensional case where the receiver time with respect to satellite system time is accurately known, the time of reception of the two satellite signals is sufficient to compute a two-dimensional position fix. Receiver 100 is in an “urban canyon” environment, surrounded by buildings 106 and 108.
As illustrated in FIG. 1, receiver 100 is able to receiver a line of sight signal 112 from satellite 104, but line of sight signals 114 from satellite 102 are blocked by building 106. Receiver 100 receives instead multipath signals, here shown as a single signal 116 from satellite 102. Since the multipath signal 116 travels a longer distance to receiver 100 than the line of sight signal would have traveled, receiver 100 records a later time for the time of arrival of the signal.
The computed position 118 of the receiver based on the time of arrival of the signals of satellites 102 and 104 occurs at one of the intersection points of two circles defined with centers at the satellites and respective radii equal to the measured distance of the receiver 100 from the satellite. Because receiver 100 measures a greater distance from satellite 102 than the actual distance to satellite 102, the relevant intersection point is calculated lower than and to the right of receiver 100's true position.
In the past, multipath mitigation strategies have focused on signal processing techniques, such as the rejection of late arriving signals or the analysis of signal shape. These processing techniques are, unfortunately, limited in their ability to accurately remove the effects of multipath from the positioning solution provided by navigation receivers. Furthermore, traditional techniques have not been able to determine whether a line of sight signal is present, or whether only multipath signals are present. Thus, improved methods, systems and devices are called for.